Petrogenesis and Significance of the Xiaodongshan Volcanic Rocks in the Aqishan-Yamansu Belt, Eastern Tianshan, Xinjiang: Constraints from Geochronology, Element Geochemistry and Sr-Nd-Hf Isotopes
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摘要: 阿奇山-雅满苏构造带是东天山造山带的重要组成部分，其大地构造属性至今仍存在争议.小东山火山岩出露于阿奇山-雅满苏构造带的西段，包括基性到酸性系列火山岩，是认识阿奇山-雅满苏构造带属性和演化的有效“岩石探针”.本次工作对其开展了系统的岩石学、锆石U-Pb年代学、元素和同位素地球化学研究.研究结果表明，小东山火山岩主要由花岗斑岩、英安斑岩、安山岩、玄武岩以及橄榄玄武岩组成，元素协变关系指示该套火山岩系列属于同源岩浆的演化产物.锆石LA-ICP-MS U-Pb测年结果显示，小东山火山岩年龄为318~308 Ma，形成于晚石炭世.该套火山岩SiO2含量变化范围较大（48.10%~76.82%），Al2O3含量整体较低且变化范围较小.从基性岩到酸性岩，稀土总量逐渐增加，Eu/Eu*值逐渐降低.中酸性火山岩大离子亲石元素Rb、K、Pb、Sr等相对富集，高场强元素Nb、Ta、Ti、P、Th等强烈亏损.而基性火山岩主要表现为Nb、Ta、Zr、Hf、Ti负异常，K、Pb、P正异常.所有火山岩均以低（87Sr/86Sr）i（0.704 50~0.707 14）、高εNd（t）（+3.16~+5.71）以及高εHf值（+5.45~+12.18）为特征.上述元素和同位素地球化学特征指示了小东山火山岩形成于岛弧构造环境，起源于交代地幔楔的部分熔融并经过了结晶分异作用.结合前人对区域构造-岩浆活动的认识，本文认为小东山火山岩所在的阿奇山-雅满苏岛弧带是在石炭纪古天山洋向南俯冲过程中形成.
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关键词:
- 岛弧 /
- 元素和同位素地球化学 /
- 小东山火山岩 /
- 石炭纪 /
- 阿奇山-雅满苏构造带 /
- 东天山 /
- 岩石学
Abstract: The Aqishan-Yamansu tectonic belt is an important part of the eastern Tianshan Orogenic Belt, while the tectonic setting and evolution of this region have long been a matter of debate. The Xiaodongshan volcanic rocks located in the west of the Aqishan-Yamansu belt, consist of mafic-intermediate-felsic volcanic association, which present an appropriate "rock probe" to further understand the Aqishan-Yamansu belt. Geochronology, element geochemistry and Sr-Nd-Hf isotope studies on the Xiaodongshan volcanic rocks have been carried out in this paper. It is found that the Xiaodongshan volcanic rocks comprise dacite porphyry, granite porphyry, andesite, basalt and olivine basalt. The correlativity of major elements indicates that this volcanic association was resulted from the comagmatic evolution. LA-ICP-MS U-Pb dating on zircons of the volcanic rocks obtained the ages of 318-308 Ma, indicating the formation of Late Carboniferous. The SiO2 contents range from 48.10% to 76.82%, whereas the Al2O3 contents are relatively low and only have a little variation. From the mafic to felsic rocks, the abundances of rare earth elements (REE) gradually increase, and the Eu/Eu* values gradually decrease. The large ion lithophile elements (LILEs) of Rb, K, Pb, Sr are relatively enriched, whereas the high field strength elements (HFSEs) of Nb, Ta, Ti, P, Th are depleted for the intermediate-acid volcanic rocks. The mafic volcanic rocks mainly show negative anomalies of Nb, Ta, Zr, Hf and Ti and positive anomalies of K, Pb and P. All volcanic rocks are characterized by low (87Sr/86Sr)i (0.704 50-0.707 14), high εNd(t) (+3.16-+5.71) and εHf (+5.45-+12.18). The above evidence indicates that the Xiaodongshan volcanic rocks originated from partial melting of metasomatic mantle wedge and fractional crystallization. Based on our new data and previous studies on the regional tectonic-magmatic evolution, we propose that the Aqishan-Yamansu tectonic belt is an island arc formed during the southward subduction of the ancient Tianshan Ocean in the Late Carboniferous. -
图 1 东天山造山带构造简图（据Zhang et al., 2021修改）
Fig. 1. Geological map of the eastern Tianshan orogenic belt and distribution of deposits (after Zhang et al., 2021)
图 2 小东山火山岩地貌特征（a）和剖面图（b）
图a镜头指向60°；剖面图中的五角星为地球化学分析采样点；三角形为年龄采样点
Fig. 2. Topographic feature (a) and cross-section (b) of the Xiaodongshan volcanic rocks
图 3 小东山火山岩野外和手标本特征
a.花岗斑岩与安山岩接触带；b.英安斑岩；c.花岗斑岩；d.安山岩；e.玄武岩；f.橄榄玄武岩
Fig. 3. Outcrops and samples of the Xiaodongshan volcanic rocks
图 4 小东山火山岩镜下显微特征（正交偏光）
a，b.花岗斑岩；c.英安斑岩；d.安山岩e.玄武岩；f.橄榄玄武岩. Qtz.石英；Cal.方解石；Pl.斜长石；Kfs.钾长石；Ep.绿帘石；Chl.绿泥石；Aug.普通辉石；Ol.橄榄石；Ser.绢云母
Fig. 4. Micrographs of the Xiaodongshan volcanic rocks (crossed polarized light)
图 5 小东山火山岩不同岩性锆石CL图像、测点位置及年龄谐和图
测点直径均为35 μm
Fig. 5. Representative zircon cathodoluminescence images, analysed spots and concordia diagrams of the Xiaodongshan volcanic rocks
图 6 小东山火山岩锆石εHf(t)-U-Pb年龄图解
Fig. 6. Zircon εHf(t) values versus U-Pb ages diagram of the Xiaodongshan volcanic rocks
图 7 小东山火山岩Na2O+K2O-SiO2图解
Ir分界线上方为碱性系列，下方为亚碱性系列岩石
Fig. 7. Na2O+K2O - SiO2 plot of the Xiaodongshan volcanic rocks
图 9 小东山火山岩的球粒陨石标准化稀土元素配分曲线(a)和原始地幔标准化微量元素蛛网图(b)
球粒陨石和原始地幔值引自Sun and McDonough（1989）
Fig. 9. Chondrite-normalized REE diagram (a) and primitive mantle-normalized spider diagram (b) of the Xiaodongshan volcanic rocks
图 10 小东山火山岩εNd(t)-(87Sr/86Sr)i图解
图中各区域数据引自Rollinson and Pease（2021）
Fig. 10. εNd(t)-(87Sr/86Sr)i diagram for the Xiaodongshan volcanic rocks
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